Vine, a service launched Thursday by Twitter that allows users to quickly shoot and post six-second video clips, has already faced criticism following users' discovery that its Facebook search feature is disabled.

The app is designed to allow users to find friends through their preexisting Twitter and Facebook accounts, but doing a friend search through the Facebook option only generates the message, "an error occurred."

At press time, Facebook had yet to definitively say whether it has intentionally blocked the feature. But on Friday the company published a clarification of its platform policies to its developer blog following a series of questions the site said it had received over the past few days.

Facebook addressed the criticism in the blog post, saying that most social apps are welcome but not if they duplicate Facebook's functionality or "bootstrap their growth in a way that creates little value for people on Facebook."

Facebook's platform policies also say that apps should not include data obtained from the social media site in any search engine or directory without its written permission.

Why Vine is disabled

Twitter said it has no comment beyond the notification that appears in the app and would not say whether the disabled Facebook search is a problem on its end. But Facebook would have plenty of reasons to block it, said Greg Sterling, senior analyst with Opus Research.

Chief among them, Sterling said, is the fact that Facebook and Twitter are competitive social networks, and it behooves both sites to be protective of their user base and, in Facebook's case, advertising revenues.

"Facebook is cautious about who gets access to its information" and would take a hard line against properties it considers competitive, he said. "It does not want to empower competitive efforts."

There are other examples of this tension between social networking sites -- Vine is the just the latest in a string of cases of apps that have been blocked due to competitive interests.

Facebook saw enormous appeal in Instagram's fervent user base when it made its $1 billion offer to buy the photo-sharing app last year. It was not long, however, before Instagram disabled its integration within Twitter after Twitter blocked access to its social graph to Instagram users.

Therefore, whether the latest development with Vine is seen as retaliatory to Twitter or whether it's part of a larger trend, it's important to consider who gets access to the API, Sterling said.

Twitter unveiled Vine as a way for users to connect through short video clips in a fashion akin to posting Twitter messages.

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